In the construction of integrated circuits, device geometries are constantly shrinking, resulting in an increase in parasitic capacitance between devices. Parasitic capacitance between metal interconnects on the same or adjacent layers in the circuit can result in crosstalk between the metal lines or interconnects and in a reduction of the response time of the device. Lowering the parasitic capacitance between metal interconnects separated by dielectric material can be accomplished by either increasing the thickness of the dielectric material or by lowering the dielectric constant of the dielectric material. Increasing the thickness of the dielectric materials is, however, contrary to the goal of reducing device and structure geometries.
As a result, to reduce the parasitic capacitance between metal interconnects on the same or adjacent layers, one must change the material used between the metal lines or interconnects to a material having a lower dielectric constant than that of the materials currently used, i.e., silicon dioxide (SiO.sub.2), k.apprxeq.4.0.
Jeng et al. in "A Planarized Multilevel Interconnect Scheme with Embedded Low-Dielectric-Constant Polymers for Sub-Quarter-Micron Applications", published in the Journal of Vacuum and Technology in June 1995, describes the use of a low dielectric constant polymeric material, such as parylene, as a substitute for silicon dioxide (SiO.sub.2) between tightly spaced conductive lines or other strategically important areas of an integrated circuit structure. Parylene, a generic name for thermoplastic polymers and copolymers based on p-xylylene and substituted p-xylylene monomers, has been shown to possess suitable physical, chemical, electrical, and thermal properties for use in integrated circuits. Deposition of such polymers by vaporization and decomposition of a stable dimer, followed by deposition and polymerization of the resulting reactive monomer, is discussed by Ashok K. Sharma in "Parylene-C at Subambient Temperatures", published in the Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 26, at pages 2953-2971 (1988). Parylene polymers are typically identified as Parylene-N, Parylene-C, and Parylene-F corresponding to non-substituted p-xylylene, chlorinated p-xylylene, and fluorinated p-xylylene, respectively. Properties of such polymeric materials, including their low dielectric constants, are further discussed by R. Olson in "Xylylene Polymers", published in the Encyclopedia of Polymer Science and Engineering, Volume 17, Second Edition, at pages 990-1024 (1989).
Parylene-N is deposited from non-substituted p-xylyene at temperatures below about 70-90.degree. C. However, the parylene-N films typically do not adhere well to silicon oxide and other semiconductor surfaces. Furthermore, the parylene-N films have poor thermal stability at temperatures above about 400.degree. C., and the films typically are not used in integrated circuits when subsequent processing temperatures will exceed 400.degree. C. Thermal stability and adhesion of parylene films is improved by fluorinating or chlorinating the dimer of p-xylylene to make parylene-F films or parylene-C films. However, the substituted p-xylylene dimers are substantially more expensive than the non-substituted dimer and are more difficult to process. The substituted dimers are typically cracked at temperatures which degrade the substituted p-xylylene monomers, and the parylene-C and parylene-F films must be deposited at temperatures substantially lower than 30.degree. C.
The problems of conventional parylene films have resulted in research regarding copolymers of p-xylylene monomers and other monomers that condense at about the same temperatures at which the p-xylyene monomers condense. Copolymerization of p-xylylene has primarily focused on monovinyl compounds (i.e., one pendent carbon--carbon double bond) to avoid addition of non-polymerized vinyl groups to the polymer. Non-polymerized carbon--carbon double bonds in the parylene-N polymers contribute to the limited thermal stability. Multivinyl monomers typically polymerize through only one vinyl group because remaining vinyl groups are not readily accessed by the same or neighboring polymerization sites. Non-substituted p-xylylene is essentially a divinyl monomer, but substantially polymerizes through vinyl groups at each end of the monomer leaving carbon--carbon double bonds in the center ring portion of the polymerized monomer. Some of the p-xylyene monomer polymerizes through only one vinyl group resulting in more remaining vinyl groups than expected. At temperatures above about 400.degree. C., the remaining vinyl groups may form reactive groups that break down the copolymer structure by a variety of mechanisms.
Copolymerization of p-xylylene is difficult to achieve since both monomers must condense on the substrate and have similar reactivity. Copolymerization of p-xylylene and monovinyl compounds has not resulted in a suitable copolymer film for integrated circuits.
As a result, there remains a need for a process for depositing a low dielectric copolymer on a substrate such as an integrated circuit, the copolymer having increased thermal stability and improved adhesion in comparison to parylene-N films, the copolymer process having controllable process conditions that are suited for integrated circuit processes in comparison to parylene-C and parylene-N films.